The present invention generally relates to material ablation with pulsed light sources and particularly relates to laser drilling and laser milling.
Material ablation by pulsed light sources has been studied since the invention of the laser. Reports in 1982 of polymers having been etched by ultraviolet (UV) excimer laser radiation stimulated widespread investigations of the process for micromachining. Since then, scientific and industrial research in this field has proliferatedxe2x80x94mostly spurred by the remarkably small features that can be drilled, milled, and replicated through the use of lasers.
Ultrafast lasers generate intense laser pulses with durations from roughly 10xe2x88x9211 seconds (10 picoseconds) to 10xe2x88x9214 seconds (10 femtoseconds). Short pulse lasers generate intense laser pulses with durations from roughly 10xe2x88x9210 seconds (100 picoseconds) to 10xe2x88x9211 seconds (10 picoseconds). A wide variety of potential applications for ultrafast lasers in medicine, chemistry, and communications are being developed and implemented. These lasers are also a useful tool for milling or drilling holes in a wide range of materials. Hole sizes as small as a few microns, even sub-microns, can readily be drilled. High aspect ratio holes can be drilled in hard materials, such as cooling channels in turbine blades, nozzles in ink-jet printers, or via holes in printed circuit boards.
The ability to drill holes as small as microns in diameter is a basic requirement in many high-tech manufacturing industries. The combination of high resolution, accuracy, speed, and flexibility has allowed laser processing to gain acceptance in many industries, including the manufacture of integrated circuits, hard disks, printing devices, displays, interconnects, and telecommunication devices. The need remains, however, for a system and method of laser drilling that solves several problems that continue to exist in the field of material ablation with pulsed light sources.
One problem that continues to exist in the field of material ablation with pulsed light sources relates to use of ultrafast lasers for parallel material ablation to address spectral dispersion issues. Due to the large spectral bandwidth of femtosecond laser sources, there can be problems if one attempts to use femtosecond laser pulses with a diffractive optical element (DOE) as a beamsplitter for parallel processing, e.g., drilling multiple holes simultaneously. This is because the DOE is spectrally dispersive which can cause focus distortion, often severely. This drawback reduces its utility as a commercial manufacturing tool. What is needed is a way to use ultrafast lasers for parallel material ablation to address spectral dispersion issues.
Another problem that continues to exist in the field of material ablation with pulsed light sources relates to control of thermal effects during ablation to improve the quality and repeatability of the holes drilled. Most current drilling techniques perform laser drilling with long pulse, high-energy lasers. The thermal effects that occur when these techniques are used cause the shape of the workpiece holes drilled to be unpredictable and not repeatable. What is needed is a way to control thermal effects during ablation to improve the quality and repeatability of the holes drilled.
Another problem that continues to exist in the field of material ablation with pulsed light sources relates to control drilling precision and the resulting hole shape during laser ablation. For many applications, such as inkjet printer nozzle holes, customers require that holes be drilled having a tapered shape where the input end of the hole is wider than the exit hole. The measurements of the hole (input diameter, exit diameter, and taper) are critical to the product quality and the operation of the end application. For example, the taper of a drilled hole controls the fluid dynamics of an inkjet printer nozzle. The hole measurements for a given product could vary widely and require the ability to adjust for a specific end application. What is needed is a way to control drilling precision and the resulting hole shape during laser ablation.
Another problem that continues to exist in the field of material ablation with pulsed light sources relates to performing parallel drilling of tapered holes. Current methods of laser drilling use excimer lasers to drill tapered holes. Excimer lasers are generally not used with diffractive optical elements as beamsplitters due to the poor spatial beam quality of the excimer laser. Parallel processing with excimer lasers normally require masking techniques to accomplish drilling of multiple holes, which significantly reduces utilization efficiency of the laser light. On the other hand, laser sources with high spatial beam quality can be focused to small beam spots without the use of projection imaging a mask onto the work piece. What is needed is a way to perform parallel drilling of tapered holes.
Another problem that continues to exist in the field of material ablation with pulsed light sources relates to performing high quality laser drilling with metal foils while minimizing thermal effects. Current methods of laser drilling use excimer lasers to drill holes in polymeric materials. Excimer lasers are generally not conducive to drilling high quality holes in metal foils because the long-duration (nanoseconds) excimer pulses can have significant melting in metal foils that leads to poor quality of the holes. What is needed is a way to perform high quality laser drilling with metal foils while minimizing thermal effects.
Parallel processing of laser-milled holes is key to increasing the throughput of, and thus the profitability of laser micromachining. Beam splitting devices such as diffractive optical elements (DOEs) are currently used in laser micromachining to divide a single beam into multiple beams to allow parallel processing. However, hole geometry requirements, and the ability to produce consistent, repeatable results are critical to the individual manufacturing application. Use of beamsplitters introduces technical challenges in maintaining the consistency and repeatability of laser milling. Thus, the need remains for a method of design and apparatus for control of multiple beam intensity distributions that solves several problems that continue to exist in the field of material ablation with pulsed light sources.
One problem that continues to exist in the field of material ablation with pulsed light sources relates to creating multiple holes or shapes in a material that meet customer requirements for hole uniformity and repeatability. Current methods for parallel laser drilling of multiple holes in a workpiece include the use of a conventional diffractive optical element (DOE) to split a single beam into multiple sub-beams. However, because typical DOEs do not produce sub-beams of equal uniformity, the laser drilling system in turn does not provide the consistent, repeatable hole geometry that is required in the marketplace. Consistency and repeatability of multiple holes, and meeting customer specifications for those holes, is critical in micromachining applications. What is needed is a way to create multiple holes or shapes in a material that meet customer requirements for hole uniformity and repeatability.
Another problem that continues to exist in the field of material ablation with pulsed light sources relates to providing beam intensity equalization in a laser drilling system. DOEs are typically designed to divide a single laser beam into multiple beams of equal intensity. However, due to design limitations and manufacturing flaws, many DOEs do not provide equal or sufficiently uniform beam intensities across all sub-beams. If a DOE is not designed to compensate for its design weaknesses, other elements in a laser drilling system must be added or changed to compensate for this deficiency. What is needed is a way to provide beam intensity equalization in a laser drilling system.
Laser milling has historically been done by moving the workpiece with a moveable stage or by moving the laser beam with a galvanometer. Galvanometer scan mirrors are used to direct laser beams for these purposes. However, a periodic wide range of motion is required to keep the mechanical parts within the galvanometer lubricated and operating smoothly. Moveable stages rely on mechanical moving stages, which are slow and thus increase the production cost and decrease the throughput of a laser drilling system. Movements required for laser micromachining, especially micro-hole drilling, are generally very small and very repetitive, and cause premature wear and failure of galvanometer scan mirrors. One solution is to use small range, high-precision scan mirrors with piezo-electric actuators, such as lead zirconate titanate (PZT) actuators. However, high speed, open-loop PZT scan mirrors typically exhibit errors associated with these types of actuators. Thus, the need remains for a method of operating a PZT scan mirror system that solves several problems that continue to exist in the field of material ablation with pulsed light sources.
One problem that continues to exist in the field of material ablation with pulsed light sources relates to performing laser drilling with repeatability and precision. Laser drilling requires that holes be drilled with pre-determined shapes and tapers according to the customer specifications of the workpiece. As technologies advance and smaller components are required for end applications, the precision and accuracy required for manufacturing these components increases. As a result, the margin for manufacturing error in laser drilling has significantly decreased. What is needed is a way to perform laser drilling with repeatability and precision.
Another problem that continues to exist in the field of material ablation with pulsed light sources relates to correcting reflection geometry effects in a PZT scan mirror with high precision. Depending on the reflection geometry of the incident beam relative to the scan mirror, a voltage applied to the actuator that drives X-direction scan produces a different deflection angle from the same voltage applied to the actuator that drives the Y-direction scan. What is needed is a way to correct reflection geometry effects in a PZT scan mirror with high precision.
Another problem that continues to exist in the field of material ablation with pulsed light sources relates to correcting hysteresis inherent in a PZT scan mirror. PZT actuators exhibit hysteresis and drift. In some applications, this is not a problem, but where accuracy and drift-free operation are required, corrections are required. Laser drilling often requires accuracy and precision to the micrometer. What is needed is a way to correct hysteresis inherent in a PZT scan mirror.
In a first aspect, the present invention is a system for laser drilling where a scan mirror and milling algorithm are used to produce high precision, controlled hole shapes in a workpiece, a picosecond laser that produces short pulses is used to reduce thermal effects to improve the quality and repeatability of the milled holes, and a DOE is used to split a single beam into a plurality of beams to allow parallel drilling of the workpiece.
In a second aspect, the present invention is a method for operating a laser drilling system where high precision, controlled hole shapes in a workpiece are drilled by using a scan mirror and milling algorithm, and by using a picosecond laser in conjunction with a DOE, thus ensuring that spectral bandwidth issues and thermal issues are addressed to improve the quality and repeatability of the holes.
In a third aspect, the present invention is an apparatus to ensure hole uniformity of holes drilled by a laser drilling system using a beam splitter, including a laser source, an efficient beam splitter, and a microfilter that produces sub-beams with essentially the same intensity as each other, where the microfilter compensates for sub-beam variability created by the beam splitter by filtering the transmissivity of the sub-beams.
In a fourth aspect, the present invention is an article of manufacture, which is a microfilter designed to equalize intensity of sub-beams emitted from a beam splitter in a laser drilling system. The microfilter includes a homogenization means to analyze and filter each sub-beam to ensure uniformity of holes drilled by a laser drilling system and meet customer specifications.
In a fifth aspect, the present invention is a method of using a microfilter to ensure uniformity of holes drilled by a laser drilling system using a beam splitter, including: designing a microfilter, fabricating a microfilter, aligning a microfilter, and ensuring that the sub-beam intensity distribution is acceptable.
In a sixth aspect, the present invention is an article of manufacture, which is a microfilter designed to equalize intensity of xe2x80x9cbinnedxe2x80x9d sub-beams emitted from a beam splitter in a laser drilling system. The microfilter includes a homogenization means to analyze and filter the binned sub-beams to ensure uniformity of holes drilled by a laser drilling system and meet customer specifications.
In a seventh aspect, the present invention is a method of designing a microfilter used to ensure uniformity of holes drilled by a laser drilling system using a beam splitter, including: determining the intensity distribution of multiple beams, calculating the transmission values of microfilter for individual beams, binning the transmission values, and designing the physical layout of the microfilter.
In an eighth aspect, the present invention is an apparatus for driving the position of a laser beam using a laser drilling system having a scan mirror with PZT actuators, where the PZT moves in the X-Y-Z plane to adjust the beam deflection, and a controller that provides instructions to the PZT to control its movement and directs the laser beam.
In a ninth aspect, the present invention is a method of controlling and correcting PZT movement including determining incident angle, determining reflection correction factors, incorporating correction factors into a tool path algorithm, and operating system with corrections to tool path.
In an tenth aspect, the present invention is a method of controlling and correcting PZT movement including controlling the PZT voltages across the PZT to modify the PZT shape/position, storing the hysteresis curve of that PZT, and using that hysteresis curve to correct hysteresis, and providing feedback to a PZT controller.
In a an eleventh aspect, the present invention is a method of operating an apparatus for precisely driving the position of a laser beam using a laser drilling system, a PZT, and a controller means including the steps of correcting for reflection geometry effects, correcting for hysteresis effects, and providing feedback to the PZT controller in order to accurately execute a tool path algorithm.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.